What Causes the Center to Collapse After Blooming in Pour-Over Coffee | A Brief Analysis of the Depression Pit Formation in Pour-Over Coffee

The Coffee Bloom Dilemma: Understanding Pour-Over Collapses

In pour-over brewing, the bloom is our "first encounter" with the coffee grounds. If a complete, even "hamburger" dome rises in the coffee bed, the groundwork for success is laid. If the bed shows no expansion at all, the beans are likely not fresh, and the resulting cup will not taste good.

There is, however, a particular case worth examining: the bed expands during the bloom pour, but soon afterward the center collapses and sinks inward. Is this normal?

Understanding the Phenomenon

First, FrontStreet Coffee will analyze why this happens. If the bed expands, the grounds are releasing gas, which rules out stale beans. The collapse that follows expansion occurs mainly because, as the bed releases gas, the gaps between particles grow larger. Once the gas is gone, this structure becomes loose. If the pressure on the center top is too great, it cannot hold such a fluffy structure, and the middle caves in.

Causes of Coffee Bed Collapse

Excessive Water Amount

Generally, bloom water should be about twice the weight of the grounds. Sometimes, though, we accidentally add too much. For example, when FrontStreet Coffee demonstrates with a light roast bean, pouring 30 ml of water in small circles over 15 g of grounds, the bed stays puffed up at 30 seconds. But when 40 ml of water is poured in small circles at the center over 15 g of grounds, the water level in the bed rises, and at 23 seconds, as the bloom liquid drains down, the center of the "hamburger" bed shows slight collapse.

Too "Aggressive" Watering

Bloom water is usually poured from the center outward in circles, and the coffee "hamburger" also expands slowly from the center to the surroundings. With fluffy grounds (such as medium-dark roast), even small center circles can make the "hamburger" expand from the center outward.

So, in terms of results, small circles and large circles do not differ much; judge by the actual situation. Gentle versus aggressive watering, however, affects the state of the bed differently.

Gentle watering does not significantly disturb the bed's structure, whereas more aggressive watering—especially when focused on small center circles—creates larger gaps between the particles directly hit by the stream. The center's support comes mainly from gas release blocking the water's downward flow. Once the gas is fully released, the water flows down, leaving a hollow bed that naturally collapses.

Coffee Grounds Too Coarse

If the grounds are ground too coarsely, water will reach the lower server before it has time to penetrate the interior of the coffee particles. When grounds first meet water, the surface releases gas and the gaps between particles expand (that is, the bloom). But if the water flows into the lower server before entering the particles' interiors, those gaps lack support and collapse easily.

Conclusion

A collapse during the bloom is clearly not normal; it reflects problems with our parameters or technique. So when we encounter a bloom collapse, we need to "prescribe the right remedy" by identifying and solving the problem through parameters and technique.

FrontStreet Coffee Recommendation

Bloom behavior is easiest to read when you are brewing a fresh, light-roast single origin, and FrontStreet's heirloom Ethiopian lots are ideal for the exercise. FrontStreet Ethiopia Flower Champion, with its tropical fruit aromas, and FrontStreet's washed Yirgacheffe, with its jasmine and bright citrus acidity, both reward careful pour control: they are clean, expressive, and quick to reveal whether your water volume, pour speed, or grind is off.

Pair them with a Hario V60. Its 60° cone and single large hole make flow speed sensitive to how you pour, which is exactly what makes it a good teaching tool for reading the bloom. Use 15 g of coffee at a 1:15 ratio and 90-93 degrees Celsius for these light roasts, and keep the bloom water to roughly twice the coffee weight—30 g for 30 seconds—before moving into a segmented pour.